Browse by

.

Pin connections

You can use the 555 effectively without understanding the function of each pin in detail. The 555 timer is an extremely versatile integrated circuit which can be used to build lots of different circuits.
.

Astable circuits
Astable circuits produce pulses. The circuit most people use to make a 555 astable looks like this:

As you can see, the frequency, or repetition rate, of the output pulses is determined by the values of two resistors, R1 and R2 and by the timing capacitor, C. The design formula for the frequency of the pulses is:

The HIGH and LOW times of each pulse can be calculated from:

The best thing to do is to rearrange the design formula so that the R values are on the right hand side:
Now substitute for R1 and f :
. the HIGH and LOW times of the pulses are approximately equal. is given by:
An alternative measurement of HIGH and LOW times is the mark space ratio:
Before calculating a frequency. with R values in kΩ and C values in µF.The duty cycle of the waveform. the frequency will be in Hz. Suppose you want to design a circuit to produce a frequency of approximately 1 kHz for an alarm application. it is often more convenient to work with other combinations of units: resistance capacitance
F
period
s
frequency
Hz
µF
s
Hz
µF
ms
kHz
With R values in MΩ and C values in µF. What values of R1. frequencies will be in kHz. However. usually expressed as a percentage. you should know that it is usual to make R1=1 kΩ because this helps to give the output pulses a duty cycle close to 50%. as already explained. This leaves you with the task of selecting values for R2 and C. Alternatively. Remember that design formulae work in fundamental units. that is. R2 and C should you use? R1 should be 1kΩ .

it is important to remember that practical values for R2 are between 1 kΩ and 1MΩ .) A test circuit can be set up on prototype board. (The E12 range tells you which values of resistor are manufactured and easily available from suppliers. the LED should flash at around 10 Hz. you must choose a value for C. or with temperature. as follows:
With the values of R1. What happens if you replace R2 with an LDR or a thermistor? This gives an astable which changes frequency in response to light intensity.
.01 µF:
that is:
and:
This is within the range of practical values and you can choose values from the E12 range of 68 kΩ or 82 kΩ . R2 and C shown. so C values will be in µF. Suppose you choose C = 10 nF = 0. To make further progress. At the same time.You are using R values in kΩ and f values in kHz.

To make things even easier. the duty cycle can be any value you want. is now filled only through R1 and emptied only through R2. C. The program works with Windows 95 and looks like this: To download the program (~210K). it is quite easy to choose appropriate values for a 555 timer astable. you might like to download the DOCTRONICS 555 timer component selection program. the duty cycle will be greater than 50% (equivalent to a mark space ratio of more than 1.0).
. The timing capacitor.0). The design equation for the output pulse frequency is:
HIGH and LOW times are calculated from:
With this circuit.Astable component selection
With a little practice. if R2 > R1. On the other hand. the duty cycle will be less than 50% (mark space ratio less than 1. If R1 > R2. Extended duty cycle astable: An extremely useful variation of the standard astable circuit involves adding a diode in parallel with R2:
This simple addition has a dramatic effect on the behaviour of the circuit. click on its image.

0).
. (This happens because the maximum voltage reached by the output pulses is less than the power supply voltage.
However. this circuit is not the one to choose. it is probable that the HIGH time will be longer than the LOW time. if you build this circuit.) Things will get worse if the output current increases. The design equation for the circuit is:
The HIGH and LOW times are supposed to be equal. including roulette. but which then slows down and eventually stops altogether. pin 3. Minimum component astable: This is a cheap and cheerful astable using just one resistor and one capacitor as the timing components:
Note that there is no connection to pin 7 and that R1 is linked to the output. can be increased using an astable circuit which is triggered to produce rapid pulses initially. giving a duty cycle of 50% (equivalent to a mark space ratio of 1. Diminishing frequency astable: The excitement and realism of electronic games. If you need an astable circuit which can be adjusted to give an accurate frequency.This version of the 555 astable is used in the cyclist/pedestrian safety lights project.

To trigger the next pulse. with the result that it takes longer and longer to charge up the timing capacitor. a 555 astable circuit functions as normal. The coasting time is determined by the 47 µF capacitor. The initial frequency can be calculated from the design equation for the basic 555 astable. the voltage across the 47 µF drops below this value and the pulses stop. the initial frequency is about 100 Hz and the output pulses coast to a stop after around 40 seconds. Experiment with different values until you get the effect you want. the voltage across C must increase to two thirds of the power supply voltage. the 47 µF capacitor in parallel with the timing network. R2 and C. you should use large values for the resistors R1 and R2. When the button is released. After a while. With the values shown. is held HIGH.It is easy to modify the basic 555 astable circuit to produce this result:
When the 'go' button is pressed. You can investigate this effect by connecting a switch/pull down resistor voltage divider to pin 4:
. output pulses are stopped. the astable continues to oscillate but the charge stored slowly leaks away. However. R1. pin 4. if the RESET input is held LOW. charges up very quickly through the 100 Ω resistor. To give a realistic coasting time.
RESET input
If the RESET input.

Under normal conditions. sounding the alarm. In an electronic die.
CONTROL VOLTAGE input
. it is impossible to 'cheat' by holding down the button for a definite length of time. Think about how you could use this circuit together with a bistable as part of a burglar alarm. or the DOCTRONICS component selector program to calculate the frequency of pulses you would expect to obtain with this circuit. If the alarm is triggered. the output of the bistable goes HIGH and the pulses start. provided the output pulses are fast enough. the output of the bistable is LOW and the astable is stopped.Here is the circuit on prototype board:
Use the design formula.

the CONTROL VOLTAGE input is not used.
In most applications. C timing network. You don't need to do this in building a test circuit. you can alter the timing characteristics of the device. of the output pulse determined?
The circuit used to make a 555 timer monostable is:
. In the circuit below. producing an output frequency which can be higher or lower than the frequency set by the R1. pin 5.
Monostable circuits
A monostable circuit produces a single pulse when triggered. In the astable mode. The CONTROL VOLTAGE input can be used to build an astable with a frequency modulated output. R2.By applying a voltage to the CONTROL VOLTAGE input. It is usual to connect a 10 nF capacitor between pin 5 and 0 V to prevent interference. one astable is used to control the frequency of a second. or period. giving a 'police siren' sound effect.7 V to the power supply voltage. the control voltage can be varied from 1. but this 'bypass' or 'decoupling' capacitor should be included in your final circuit. The two questions about monostables you immediately need to ask are:
• •
How can the circuit be triggered to produce an output pulse? How is the duration.

must be of shorter duration than the intended output pulse. by a sudden transition from HIGH to LOW. The period.1 s. You can build a test version of the 555 monostable as follows:
. produced by pressing the button. of the output pulse can be calculated from the design equation:
Remember again about compatible measurement units: resistance capacitance F µF µF period s s ms
With R1 = 1 MΩ and C = 1 µF.As you can see. that is. The trigger pulse. The circuit is triggered by a falling edge. τ . the output pulse will last for 1. the trigger input is held HIGH by the 10 kΩ pull up resistor and is pulsed LOW when the trigger switch is pressed.

the 555 component selection program can be used to investigate the effect of different R1 and C values:
More about triggering
For a simple 555 monostable. the trigger pulse must be shorter than the output pulse. Sometimes you want to trigger the monostable from a longer pulse:
.By clicking on the monostable tab.

loudspeakers and small motors can be connected directly to the output of the 555. The period of the monostable pulse is shorter than the period of the Vin pulses.The trigger network detects the falling edge at the end of each Vin pulse. as an electronic switch which turns the transducer ON or OFF:
. This means that output transducers including buzzers. it is interesting to investigate the action of the trigger network 555 as a transducer driver
A transducer is a subsytem which converts energy from one form into another. In an output transducer. You can use the 555 as a transducer driver. sound. filament lamps. that is. electrical energy can be converted into light. pin 3. for example. or movement. producing a short 'spike' which triggers the monostable at the appropriate time. If you want to trigger the monostable from a rising edge. where one of the forms is electrical. The output of a 555 timer can deliver more than 100 mA of current. you need to add a transistor NOT gate to the trigger circuit:
If you build these circuits.

This circuit has an inverting Schmitt trigger action.) If a filament lamp is connected between the positive power supply rail and the output. and when Vin is HIGH. (If an input change in one direction produces a different result from a change in the opposite direction. but the lamp will light when the input voltage is LOW:
. current flows through the lamp when the output voltage is LOW. the output is LOW. the output is HIGH. as shown above. If you connect the lamp between the output and 0 V. Once this level has been exceeded. If Vin is slowly increased starting from 0 V. decreasing Vin does not affect the output until Vin drops below 1/3 of the power supply voltage. the output voltage snaps from HIGH to LOW when Vin reaches a level equal to 2/3 of the power supply voltage. In other words. The 'inverting' part of this description means that when Vin is LOW. the lamp lights when the input voltage is HIGH. the circuit is said to show hysteresis. the circuit will still work. In a 'Schmitt trigger' circuit there are two different switching thresholds.

with a HIGH/LOW digital input signal the same result can be achieved more obviously and at lower cost using a transistor switch circuit. pin 4 is left unconnected.
. The circuit can be simplified by omitting the 10 nF bypass capacitor. and will continue to work when the RESET input. However. Some people are very fond of this circuit and use it whenever a transducer driver is required. in both versions of the circuit pins 2 and 6 are joined together.Note that.